I have an oracle database 9iR2 installed in my server (32 bits processor, 4GB ram). And I want to add more RAM for oracle database. It is possible? and do I need to do any setting for the previous oracle instance to fit the newly added RAM (Swap space)?

yes, you can add more RAM without any problem. Take into account that if the O.S. it's windows 32b. , it will only see 3,3 GB (unless using PAE). Linux OS has no problem with that.

You don't have to make any previous settings or changes in the database. Just add RAM. After you've added the RAM, the database just will continue using the same amount as before. You'll have to change this modifying all the memory related parameters (SGA, ...)